A longstanding objective within the materials, engineering, biomedical and analytical sciences has been the design of ever-smaller structures and devices for use in miniature systems capable of performing specific functions, such, as sensors, transducers, signal processors or computers. Of particular interest as potential building blocks in this context have been functional materials having predetermined properties. Patterned films composed of suitable polymers and polymer-microparticle composites offer particularly attractive opportunities to realize hierarchically organized structures of functional materials and to provide confinement and segregation for performing “local” chemical reactions.
Several methods of preparing patterned polymer films and polymer-microparticle composites have been described. In one example, polymer molding has been used to prepare polymeric films. Beginning with a master that is fabricated from a silicon (Si) wafer using conventional lithographic techniques, a mold is made using an elastomer such as polydimethylsiloxane (PDMS). The mold is then used to produce replicas in a UV-curable polymer such as polyurethane. The applicability of this technique of polymer molding, long used for replication of micron-sized structures in devices such as diffraction gratings, compact disks, etc., recently has been extended to nanoscale replication (Xia, Y. et al., Adv. Mater. 9: 147 (1997), Jackman, R. J. et al., Langmuir. 15:2973 (1999), Kim, E. et al. Nature 376, 581 (1999).
Photolithography has been used to produce patterned, stimuli-sensitive polymeric films which can be further functionalized with bioactive molecules and which undergo abrupt changes in volume in response to changes in pH and temperature (Chen, G. et al., Langmuir. 14:6610 (1998); Ito, Y. et al., Langmuir 13: 2756 (1997)). UV-induced patterned polymerization of various hydrogel structures within microchannels has been described as a means for the autonomous control of local flow (Beebe, D. J. et al., Nature. 404:588 (2000)).
Surface-initiated ring-opening metathesis polymerization following microcontact printing has been used to create patterned polymer layers which remain attached to the surface and produce structures of controlled vertical and lateral dimensions (Jeon, N. L. et al., Appl. Phys. Lett. 75:4201 (1999)). Other techniques such as thermal radical polymerization (Liang, L., J. Appl. Polym. Sci. 72:1, (1999)) and UV-induced polymerization (Liang, L., J. Membr. Sci. 162:235 (1999)) have been used to generate surface-confined, thin, uniform and stimuli-sensitive polymeric films.
Sarasola, J. M. et al. (J. Electroanal. Chem. 256:433, (1988)) and Otero, T. F. et al. (J. Electroanal. Chem. 304:153, (1991)) describe electropolymerization of acrylamide gels using a Faradaic process. Acrylamide gels are prepared on electrode surfaces by an anodic oxidative polymerization process using the electroactive nature of acrylamide monomers.
Polymerization of crosslinked acrylamide has been reported to produce a matrix of glass-immobilized polyacrylamide pads which were activated with receptor molecules of interest including oligonucleotides or proteins. The use of the resulting porous and highly hydrated matrix for simultaneous monitoring of ligand-receptor binding reactions has been reported (Proudnikov, D. et al., Anal. Biochem. 259:34 (1998); Yershov, G., Proc. Natl. Acad. Sci. U.S.A. 93:4913 (1996), LaForge, S. K., Am. J. Med. Genet. 96:604 (2000); Khrapko, K. R. et al. U.S. Pat. No. 5,552,270, 1996; Ershov,G. M. et al. U.S. Pat. No. 5,770,721, 1998; Mirzabekov et al. U.S. Pat. No. 6,143,499). It should be noted, however, that a potential drawback of the methodology used in these studies is that forming the gel-matrix for the assay is labor-intensive and difficult, especially if a densely packed matrix is desired. Additionally, when the gel-pads of the matrix have sizes on the length scale of microns, it is a considerable technological challenge to deliver the bioactive molecules reproducibly and reliably to each gel-pad in the array.
A process for the assembly of a 3-D array of particles has been reported which is based on the synthesis of a core-shell latex particle containing a core polymer with a glass transition temperature significantly higher than that of the shell polymer. In accordance with that process, particles were assembled into a 3-D close packed structure and annealed in such a way that the core particle remained unaltered while the shell polymer flowed, resulting in a continuous matrix embedding an organized 3-D array of core particles (Kalining, O. and Kumacheva, E., Macromolecules. 32:4122 (1999); Kumacheva, E. et al., Adv. Mater. 11:231 (1999), Kumacheva, E. et al., U.S. Pat. No. 5,592,131 (1999)). However, the reported assembly of the 3D array is quite slow because it relies on particle sedimentation. Second, because the outer shells of the particles are destroyed as a result of annealing, the particles cannot be reused.
The encapsulation of a colloidal crystalline array within a thin, environmentally sensitive hydrogel matrix capable of swelling in response to changes in pH and temperature has also been reported. In other instances, the hydrogel contained immobilized moieties capable of triggering the swelling of the gel in the presence of particular analytes. The swelling of the gel matrix increases the periodicity of the colloidal crystal array and produces a shift in Bragg diffraction peaks in the spectra of the scattered light (Holtz, J. H. et al., Anal. Chem. 70:780 (1998); Hacke, G. et al., U.S. Pat. No. 5,266,238, 1993; Asher, S. A., U.S. Pat. No. 5,281,370, 1994). In most of these references, the process of forming a colloid crystal relies on passive diffusive transport of particles within the prepolymer reactive mixture, which tends to be slow. In one reference, however, a process was reported in which an electric field was applied to a colloid suspension to increase the rate of formation of a colloid crystal. It should be noted that, regardless of whether an electric field is used, the processes reported in these references only produce a simple colloid crystal. More sophisticated colloid crystal structures, such as patterned two-dimensional colloid crystals, are not readily produced by these methods.
Each of the aforementioned references are incorporated herein by reference in its entirety